Cheap as chips !!
From the Amazon blurb:
WD Red hard drives are designed and tested for compatibility in the unique 24x7 operating environment and demanding system requirements of home and small office NAS.
Western Digital claims that these tweaks result in a 35 per cent MTBF (mean time between failures) improvement over standard desktop drives, and is quick to point out that the optimisation ultimately reduces the long-term cost of NAS ownership